The demands of an industrialized society and the consequent need to burn fuel for the generation of energy tend to compromise air quality. Existing fuels that are burned in boiler systems and furnaces to produce heat and power include distillate fuel oil, residual (number 6) fuel oil, blends of distillate and residual fuel oil, and coal. Many of the existing energy sources, particularly fossil fuels such as petroleum and coal, release substantial amounts of pollutants, such as nitrogen oxides (NOx), sulfur oxides (SOx), carbon monoxide (CO) and particulate matter (PM) upon combustion. These pollutants cause respiratory diseases and other human ailments. These pollutants also poison the environment via acid rain, ground level ozone and greenhouse gases.
Being the least expensive and available in relative abundance, coal is the preferred fuel in many instances. Unfortunately however, the combustion of coal generates substantial quantities of harmful pollutants, such as sulfur dioxide, oxides of nitrogen, carbon monoxide and particulates. Fuels other than coal often have fewer effects on air quality, but tend to be more expensive, or be in relatively short supply.
Environmental legislation, such as the Clean Air Act in the United States, has been enacted in many countries to control the amount of various chemicals released into the atmosphere in an effort to protect human health and the environment. At a local or regional level, industry is typically regulated by state or regional environmental protection agencies that set limits as to the amounts of airborne pollutants that can be emitted from a given source.
As energy demands increase, the pressures, conflicts and costs involved in supplying that energy without exacerbating these health and environmental problems become increasingly difficult.
In recent years, much work has been conducted on finding ways of reducing the output of NOx and SOx and other pollutants from coal combustion. The most common means of reducing NOx emissions is the use of Selective Catalytic Reduction (SCR) technology. However, this approach requires the construction of an expensive catalytic system to treat the exhaust gases from the combustion process. This approach may not be cost effective for smaller coal fired furnaces or boilers.
Another approach is to use staged combustion to reduce NOx wherein there are more than one individualized combustion zones in the furnace. While this technique is suitable for the construction of a new furnace, it is not easily or inexpensively retrofitted to existing furnaces.
Yet another approach is the use of additives during the combustion process to effect changes in the combustion conditions such that emissions are reduced. In one method, calcium compounds such as CaO, CaCO3, CaF, etc, are added to the combustion zone to effect the reduction in NOx and SOx emissions. However, this and similar techniques result in a slag disposal issue.
Another method of reducing NOx emissions (at the point of exit from the stack) is to combine another fuel having less NOx emissions per heating unit value, such as fuel oil, in a furnace or boiler, with the coal. Such combination can reduce the overall NOx emissions while providing the same or similar overall heating values. Although this method often permits coal-burning facilities to meet air quality regulations, it is simply a substitution of a more expensive, but less NOx emitting fuel for the coal. The fuel oil does not actually reduce the emissions of the coal itself, but rather, reduces the emissions from the heating system itself.
In short, notwithstanding previous attempts to solve the emissions problems associated with the combustion of fossil fuels, especially coal, there is a continuing need to develop coal burning methods that improve the NOx emissions, can be cost-effectively retrofitted to existing heat-producing units, such as furnaces and boilers.